The present invention relates to a method for making fiber boards, especially asbestos-cement boards, wherein a felt band covered with a fiber fleece is fed to a pair of pressure rollers in nip relationship, the covering of the felt band by the fiber fleece taking place by deposition on the felt band and by suction from the other side of same.
As regards a prior art method of this kind, provision is made for only a single wet deposition box with one suction head for attracting the fiber fleece to the felt band.
This prior art method suffers from the drawback that because of the single vacuum head, the fiber fleece cannot be subjected to sufficient dehydration, and further adequately constant layer thickness is not feasible.
As regards another prior art method, the deposition of the fiber fleece on the felt band takes place not by wet deposition, but rather by means of a sieve cylinder rotating in a sieve box containing the mixture of water and asbestos-cement.
This latter prior art method suffers from the drawback that the sieve cylinder accepts only relatively low concentrations of solids, so that the mixture of water and asbestos-cement (the asbestos-cement sludge) must be relatively thin in the sieve cylinder box. This allows the fibers of the asbestos-cement sludge to align themselves in the direction of motion when they pass through the sieve cylinder, and therefore asbestos-cement boards or products are obtained which are of anisotropic strengths. The thinness in this latter prior art method also is determined among other causes by the relatively small drying arc of the sieve cylinder.
Another drawback of this latter prior art method consists in the tendency of a thin asbestos-cement sludge tending to precipitate, so that this un-mixing must be prevented by stirrers mounted in the sieve cylinder box. These stirrers must rotate at relatively high rpms and thereby generate turbulence in this box.
Such turbulence disadvantageously leads to uneven thickness of the layer formed in the sieve cylinder and hence to uneven thicknesses and/or densities in the boards so made.
The unevenness of the fabricated boards also extends to their structures because the turbulence generated by the stirrers leads to flocculations in the asbestos-cement sludge.